Loud-speaker baffle with elongated aperture for the egress of sound



July 28, 1953 R. CHAPMAN ETAL 2 6 6 8 LOUD-SPEAKER BAFFLE WITH ELONGATED APERTURE FOR THE EGRESS OF SOUND 3 Sheets-Sheet 1 Filed Sept- 14, 1948 Na m m mm m w A PH 5 a 5B July 28, 1953 R. CHAPMAN ETAL LOUD-SPEAKER BAFFLE WITH ELONGATED APERTURE FOR THE EGRESS OF SOUND 3 Sheets-Sheet 2 Filed Sept. 4. 1948 Attorney ly 2 1953 R. CHAPMAN ET AL 2,646,851

LOUD-SPEAKER BAFFLE WITH ELONGATED APERTURE F OR THE EGRESS OF SQUND Filed Sept. 4, 1948 3 Sheets-Sheet 3 I Attorney Patented July 28, 1953 LOUD-SPEAKER BAFFLE WITH ELONGATED APERTURE FOR THE EGRESS OF SOUND Roger Chapman and Robert Henry Trier, London, England, assignors to International Standard. Electric Corporation, New York, N. Y., a corporation of Delaware Application September 4, 1948, Serial No. 47,824 In Great BritainSeptember 10, 1947 3 Claims. (Cl. 1s1-s1) This invention relates to devices for coupling loudspeakers for the output of sound suchas bafiies.

An object of the invention is to providea loudspeaker batfle which effectively diffuses the radiation of sound waves emitted by a loudspeaker diaphragm.

Another object of the invention is to broaden the radiation pattern of the output of a loudspeaker.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of the embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 shows a loudspeaker in a box baiiie with an aperture in the form of a single slot extending the full width of the speaker diaphragm.

Fig. 2 shows a loudspeaker in a box baffle with an aperture in the form of a single slot less in length than the diameter of the speaker diaphragm.

Fig. 3 shows a loudspeaker in a box baffle with a round aperture of smaller diameter than the diameter of the speaker diaphragm.

Fig. l shows a radio set having the loudspeaker mounted behind a bafiie having two adjacent parallel slots divided by a decorative rib.

Fig. 5 shows a loudspeaker in a box baiile having two parallel slots divided by a vane.

Fig. 6 shows a loudspeaker in a box having a number of holes in lieu of slots.

According to one of its features the invention consists of a device for coupling a loudspeaker to the atmosphere for the output of sound comprising a member adapted to be placed over one side of the loudspeaker having an aperture or apertures for the egress of sound, at least one dimension of which, if there is only one aperture, or where there are more than one aperture the aggregate dimensions of all of them in at least one direction, including the width of the space or spaces between apertures, is between 1 and 3% inches and is never more than half of the diameter of the effective radiating portions of the diaphragm of the loudspeaker at 500 C. P. S.

According to another feature the invention consists of a battle for a loudspeaker with an aperture for the egress of sound, in the form of a virtual slot of the order of 3 to 4 inches in length and of the order of 1 /2 inches in width.

According to another feature the invention consists of a baflfle for a loudspeaker with an aperture for the egress of sound in the form of a virtual slot of the order of 5 to 8 inch-es in length and 1% to 2 /2 inches in width.

According to another feature the invention consists of a bafile for a loudspeaker with an aperture for the egress of sound in the form of a virtual slot of the order of 10 inches upwards in length and 1 /2 inches to 3 /2 inches in width.

According to another feature the invention consists of a baffle for a loudspeaker having an aperture for the egress of sound, in the form of a virtual slot having dimensions of the order of l to 3% inches in directions at right angles.

According to another feature the invention consists of a baffle for a loudspeaker having an aperture for the egress of sound in the form of a virtual slot of the order of 5 inches up to 18 inches in one direction and of the order of 1 /2 to 3 /2 inches in the direction normal to the first direction.

According to another feature the invention consists of a baflle for a loudspeaker having an aperture or apertures for the egress of sound from one side of the loudspeaker in the form of a slot or slots the width of which, if only one, or the aggregate width of which if more than one including the width of the space, or spaces between the slots, is not greater than 2 inches.

A loudspeaker of the so-called direct radiator type is usually mounted in a device of some kind (hereinafter referred to as a coupling device), designed to separate sound waves generated on one side of the diaphragm from sound waves generated on the other side of the diaphragm. Without a device of this sort, sound waves at the lower end of the band of audible frequencies tend to be cancelled out owing to the output from the two sides of the diaphragm being in antiphase.

The degree of separation depends on the relation of the wavelength of the sound to the length of the sound path between back and front of the loudspeaker. Coupling devices of this type take various forms such as the familiar bafile, which may be a fiat board or an open-backed box such as a domestic broadcast receiver cabinet. Alternatively various forms of enclosure or semienclosures of one side of the loudspeaker (generally the back) may be used, these alternative devices having a number of objects: for instance, the phase shifting of the sounds originating on one side of the loundspeaker so that at certain frequencies, they reinforce, instead of cancel, the sounds originating on the other side; or the modification of'the acoustic impedance of the loudspeaker system as a whole, generally with a view to improvement of bass response and the damping of bass resonances.

As these alternative devices also separate the radiation from the two sides of the loudspeaker over substantial parts of the range of frequencies reproduced they are to this extent similar to a baffle, and the word bafiie is to be understood to include such devices where it is used hereinafter in this specification.

It has been the common practice in such coupling devices to have an aperture for the egress of sound, on one side of the loudspeaker (generally the front) which is of substantially the same size as the diaphragm itself through this aperture i may be restricted somewhat by ornamental devices such as frets, used to disguise the hole.

The radiation pattern from a loudspeaker mounted in this way tends to become directional,

that is to say, the sound intensity along the axis of the loudspeaker is greater than that at an angle to the axis, for frequencies whose wavelength is comparable with or less than the dimensions of the diaphragm. This phenomenon is commonly called focussing and it becomes more marked the higher the frequency.

It has been found that focussing is considerably reduced if this aperture for the egress of sound is restricted in the manner hereafter described.

In the simplest form of the invention, the said aperture takes the form of a slot which may be substantially as long as the width of the diaphragm in one direction and smaller than that width in the other direction. Diffusion takes place in the direction of the narrow dimension, 1. e., if the slot has its longitudinal axis vertical, the diffusion is in a horizontal plane, which is the most useful arrangement in the case of a domestic broadcast receiver for instance.

The reason for the diffusion is believed to be that the slot acts as a line source radiator of sounds whose wavelength is equal to or greater than the width of the slot, the diffusion falling oif at frequencies whose wavelength is less than the slot width.

The longer dimension of the slot can be reduced so that it is less than the width of the diaphragm, and if this is done there is some diffusion in the direction of this longer dimension also but the frequency band over which this diffusion is effective is limited at the upper end of the frequency spectrum, by the length of the slot.

The shortening of the slot can be carried to the extent of making both dimensions of the aperture equal (e. g. a round or square hole) and this produces hemispherical diffusion.

There is a limit to the extent to which the area of the aperture can be reduced, however, as there is an increasing loss of sound output, principally in the middle and bass register as the area is reduced. This is believed to be due to increased air loading of the loudspeaker diaphragm by the partially confined space in front of it.

Apertures with more than one of their dimensions less than the width of the loudspeaker diaphragm have their uses where diffusion is required in more than one plane but the need to keep the loss of output within reasonable limits and the consequent necessity of maintaining a reasonable total area of aperture restricts the upper limit of sound frequency for which effective diffusion can be obtained and the usefulness of such arrangements will be limited to specialised applications such as speech reinforcement installations.

Hence in the preferred form of the invention a slot extending substantially the full width of the diaphragm is used with a width of the order of the wavelength of the highest frequency for which the maximum diffusion is required.

For full difiusion up to 7 kc./s., for instance, this width is 1.92 inches, and 1.32 inches up to the 10 kc./s.

In the application of the invention in practice, it is desirable, of course, to use a width of slot which is equal to the wavelength of the highest frequency sound to be reproduced and with the normal radio set, a top limit of reproduced frequency of say 8 kc., may be adequate. A slot of 1.65 inches, or, say, 1% inches wide will give maximum diffusion up to 8 kc.

High fidelity reproduction however demands a higher frequency range in the upper register and this calls for a narrower slot and the loss of middle and lower frequencies becomes more pronounced as the slot is narrowed.

As explained previously, if a slot is provided which is wider than the Wave length of the highest frequency sound to be reproduced, the diffusion effect will fall off at frequencies whose wave length is less than the slot width.

It is clear, therefore, that a compromise may be desirable, between high frequency diffusion on the one hand and middle and bass response on the other hand. It may well be found that the best overall results'are obtained by sacrificing some of the diffusion effect at the upper end of the frequency register for the sake of better middle and bass response.

In a domestic broadcast receiver for instance, the slot might be 2" wide with a falling off of the diffusion efiect from and above a frequency of somewhat below '7 kc./s.

The degree of decay of the diffusion effect at frequencies whose wavelength is less than the slot width has not been precisely determined but it is estimated, however, that a noticeable degree of diffusion will still be found at frequencies of the order of half an octave above the frequency of sound whose wavelength is equal to the slot width.

Thus, as an extreme example of a compromise, a slot width of 2 inches is equal to the wavelength of sounds at 5280 cycles per second, above Which the diffusion effect would diminish, but would still be noticeable up to frequencies of the order of 7500 cycles per second.

Instead of a single slot it is possible to use a plurality of parallel slots and this is equivalent, so far as the diffusion effect is concerned, to a single slot of a width equal to the distance between the outer margins of the slots, or of the outer slots, if there are more than two.

The performance of the plurality of slots will generally be somewhat inferior to that of the equivalent single slot, however, since the obstructed space between the slots, while not materially affecting the diffusion effect, nevertheless reduces the total aperture area and causes some loss of sensitivity.

A row or rows of holes can be used in lieu of a continuous slot or slots with results approximately the same as those obtained with a single slot of the same width as the holes, or the same as the distance between the outer margins-of the holes of the row or of the outer rows where there 1 are more than two rows.

fusion opposite the gaps between theholes, an effect which "is more or less important according to whether the holes are spaced wide apart or near together.

It is possible to have two slots with their longitudinal axis at right angles and while, generally speaking, this defeats the objects of the invention, in so much as the width of one slot is effectively increased at the point where it is crossed by the other slot so that diffusion in two'planes is not achieved, as might be expected. It is nevertheless possible to have a slot designed on the basis above described, crossed by a slot of substantially narrower width without seriously affecting the performance of the first slot. In fact, the same may be said of any combination of a slot designed as described above, and other apertures in addition, provided that the aggregate area of such other apertures is small in relation to the area of the slot. All such crossing slots or other aperture will tend to detract from the diffusion effect produced by the slot, but in so far as it may be desirable to use such arrangements for structural, decorative or other reasons, it is .to be understood that they are within the scope of the invention.

It is also possible, to use two slots, each of a width substantially equal to the wavelength of sounds at the frequency when maximum diffusion would be ideally desirable and to separate them by a vane in a plane parallel with the length of the slots and normal to the face of the bafile, so that the diffusion from each slot is confined to a zone on one side of the vane. The overall width between the outer margin of the two slots does not govern the upper limit of frequency to which diffusion extends, as each acts as a separate radiator. There will be some interference between the output of the two slots however as a vane of practical size will not perfectly separate the output from the two slots into two diffusion zones, but this method may be used to obtain a compromise between a useful degree of diffusion and a reduced loss of output in the middle and lower frequency ranges.

The embodiments of the invention shown in the accompanying drawings and now to be described, illustrate the various forms of the invention outlined above.

In the drawings corresponding items are denoted by the same reference numbers.

In Fig. 1 which illustrates the simplest form of the invention, a box baffle I has a slot 2 in its front panel 3 behind which a loudspeaker is mounted, the lines 4 showing the position of the diaphragm mounting ring, the speech coil 5 being shown partly by dotted lines, and the cone diaphragm 6 being shown by shading lines. The slot 2 extends across the width of the diaphragm '5 of the speaker and with this arrangement the diffusion attributable to the slot covers a space of half cylindrical form, the axis of the cylinder running down the slot vertically and the axial length of the cylinder bein co-extensive with the length of the slot.

vFig. 2 is similar to Fig. 1 except that the slot does not extend the full width of the speaker diaphragm. The diffusion attributable to the slot is in substantially cylindrical form but the cylinder may be considered as being terminated by planes normal to its axis at the intersection of such planes with the plane of the baffle front and at an angle to the bafiie front being tilted upwards at the top of the cylinder and downwards at the bottom of the cylinder. There will be considerable loss of output with this arrangement.

In Fig. '3 around hole 2 :smaller than the diaphragm is used instead of the slot and here the diffusion attributable to the .slot will be hemispherical in form. The loss of outputwill be even greater than with the arrangement of Fig.2.

Fig. 4 shows a table model broadcast receiver in 'whichtwo slots 2 :are used which'extend across the full width of the diaphragm of the speaker.

The distance between the outer margins of the two slots is equal to that of a single slot of comparable performance the division between them being a rib I which may .be added for decorative or structural reasons. Two similar ribs '8, at the outer margins of the slots may be added as a further decorativeor structural feature. A grille is shown covering the slot 2. This figure is an example of the use of a plurality of parallel slots .and, :of course, more than two slots, or more than one dividing member in a single slot, could be used.

Fig. 5 illustrates the arrangement where two parallel adjacent slots are separated by a vane. The two slots 9 and I U have between them a vane 11 parallel to the length of the slots and normal to the battlefront 3.

The perspective chosen for Figure 5, illustrates the fact that at angles where both slots can be seen, there will-be radiation from both, reaching the bearer, and if at such a listening position there is a difference between the length of the sound paths from the slots which is comparable with the wavelength of the sound, sounds around such frequencies will suffer some interference which will cause irregular diffusion. The further the vane extends from the bafile front, the less likely is this to happen.

The slots radiate into quarter-cylindrical zones each similar to the half-cylindrical diffusion zone in the case of Figure 1, and the two zones are separated by a vane H, so that each slot can be as wide as the wavelength of the highest frequency sound for which maximum diffusion is required. As there are two slots the air loading of the diaphragm is halved, enabling the diffusion to be maintained to a higher frequency for a given .loss of output in the middle and bass register as compared with the single slot of Figure '1. Alternatively the same diffusion characteristic can be maintained and the loss of output in the middle and bass register can be reduced.

Figure 6 shows a bafile with a number of holes 12 in lieu of a slot or slots which may be resorted to for decorative or structural reasons though generally speaking the results will be inferior to those obtained with plain slots, and there will be irregularities in the radiation pattern due to the spaces between the holes.

The following results (which illustrate the advantage of the invention) were obtained with a normal 10 moving coil loudspeaker.

The loudspeaker was first mounted in a plane battle with a round hole substantially the same size as the effective diaphragm radiating area and measurements of output at various frequencies were taken, at different angles to the speaker axis. Pronounced focussing became apparent at about 3 kc/s. being of the order of 8-10 db fall in output between measurements along the speaker axis and measurements at points 30 off-axis, and of 18 to 20 db at 45 off-axis. The focussing effect increased with rising frequency until at about 7 kc./s., the fall in output, as compared to the output measured along the axis,

7 amounted to 16-24 db at 30 off-axis and a little more at 45 off-axis.

Measurements of the same loudspeaker behind a vertical slot extending longitudinally the full width of the effective diaphragm radiatin area and 1%.; inches wide, showed not more than 3 or 4 db difference between measurements along the axis and measurements at various angles up to 60 off-axis, in the horizontal plane, up to frequencies of the order of 8 kc./s. above which the loudspeaker tested had a fairly pronounced cut-off.

Comparisons between output measurements along the axis, with and without the slot diffuser showed in the latter case a rising characteristic from 1 kc. up to about 5 kc. and a slight falling off from 5 kc./s. to 8 kc. This rising and substantially maintained response with rising frequency is attributed to the concentration of radiation along the axis. With the diffuser in position the rise between 1 kc. and 5 kc. is much reduced and the lower level at 5 kc. is substantially maintained up to 8 kc.

Below 1 kc. there is a slightly lower output with the diffuser slot in position and a bass resonance at about 70 cycles per second is reduced in amplitude and sharpness and lowered in frequency to about 50 to 60 cycles per second.

The net results of adding the diffuser when listening at a point along the speaker axis are:

(i) An apparent reduction in high frequency response which is the inevitable result of diffusion and is, on the whole desirable (being amply compensated by the increased output in the upper register by the use of the slot, even at quite small angles off axis) (ii) A reduced response in the middle register (between about 1 kc. and about 3 kc.), which is the price paid for the benefits of the invention; and

(iii) A less pronounced falling off in the base, which is amply compensated by a damping and lowering of the bass resonance, and this is a subsidiary advantage of the invention.

With very small cones (e. g. three inches in diameter) the invention would be useful if at least one dimension of the aperture was small compared with the effective loudspeaker diameter (e. g. 1 inches or less).

With medium size cones (e. g. from about 5 to 8 inches efiective diameter) such as are used in the normal range of domestic broadcast receivers and gramophone reproducers, the slot width will tend to be made wider than theoretical diffusion considerations would dictate, in order to avoid loss of power in the middle of the frequency range. Thus whereas design for an upper frequency limit of say 8000 C. P. S. would call for a slot about 1 inches wide, the slot may actually be made between-sayl and 2 inches wide.

With larger cones, say between 12 and 18 inches in diameter, focussing of the upper register becomes noticeable at lower frequencies and it may well be worth while to use slot widths up to 3 inches, giving maximum diffusion at frequencies of the order of 4000 C. P. S. and some useful diffusion up to about 6000 C. P. S. Since cones of this size will have a poor frequency response in any event at the latter frequency they will only be used alone where such an upper frequency limit is adequate, for instance in speech reinforcement systems, or where other measures, such as a separate high note loudspeaker, are adopted to handle the upper end of the frequency range.

The slot length may also vary from the effective cone diameter down a length equal to the designated slot width resulting, in the extreme cases, in a square, circular or polygonal aperture.

The term virtual slot used in the specification and the claims applies to any single aperture or slot of the type described, whether or not the same is obstructed or intersected by fabric, deco rative member or members or the like, and to a plurality of apertures used in lieu of a single slot. References to a dimension of a virtual slot mean the gross dimension ignoring any such obstruction or intersections, and including the spaces between a plurality of apertures, in the line of measurements; and where the dimension in a given direction is not constant for all parallel lines of measurement in that direction the largest dimension in that direction is meant.

While we have described above the principles of our invention in connection with specific em bodiments, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention.

We claim:

1. A loudspeaker comprising a diaphragm, a baffle plate arranged in front of said diaphragm and substantially perpendicular to the axis of the speaker, an elongated aperture in said baffle plate, one dimension of said aperture extending substantially the full width of said diaphragm and the other dimension of said aperture being substantially uniform and of the order of the wavelength of the highest frequency for which maximum diffusion is required.

2. A loudspeaker comprising a diaphragm, a bafile plate arranged in front of said diaphragm and substantially perpendicular to the axis of the speaker, a virtual elongated slot in said baffle, the width of said slot being substantially uniform and of the order of the wavelength of the highest frequency for which maximum difiusion is required.

3. A baffie plate for a loudspeaker arranged substantially perpendicular to the axis of the speaker comprising a plurality of apertures in the form of an elongated virtual slot, the width of said slot being substantially uniform and of the order of the wavelength of the highest frequency for which maximum diffusion is required.

ROGER CHAPMAN. ROBERT HENRY TREE.

References Cited in the file of this patent V UNITED STATES PATENTS Number Name Date 272,866 Drawbaugh Feb. 27, 1883 1,616,474 Stehlin Feb. 8, 1927 1,865,735 Wolff July 5, 1932 2,214,393 Wilbur Sept. 10, 1940 2,373,692 Klipsch Apr. 17, 1945 FOREIGN PATENTS Number Country Date 404,937 Great Britain Jan. 15, 1934 

